OCaml increment tuples recursively in order and add to list - Compiler error

increment-tuples.ml

(*
   I am attempting to create a 2d 'map' for a simple ncurses game.
   The map should consist of a list of coordinates (tuples of int * int)
   For example: [(0,0);(0,1);(0,2);(1,0);(1,1);(1,2);(2,0);(2,1);(2,2);]
   would be the list that represents all coordinates in the 3x3 'map':

   2###
  y1###
   0###
    012
     x

   To create this list of coordinates, I wrote a function that takes
   the minimum and maximum values for both x and y in addition to a
   list to be populated with the resulting values.

   See code and comments below
*)

(* Example output list of tuples *)
let list_of_incremented_tuples = [(0,0);(0,1);(0,2);(1,0);(1,1);(1,2);(2,0);(2,1);(2,2);]

(* Recursive function that now works *)

let rec add_incremented_tuples_to_list x x_max y y_max l =
    let l = if y <= y_max then
        add_incremented_tuples_to_list x x_max (y + 1) y_max ((x, y) :: l)
    else l in
    let l = if x <= x_max then
      add_incremented_tuples_to_list (x + 1) x_max y y_max ((x, y) :: l)
    else l in
  l

(* Example execution *)
let list_of_incremented_tuples = add_incremented_tuples_to_list 0 2 0 2 []

(* val list_of_incremented_tuples : (int * int) list =
     [(3, 2); (3, 1); (3, 0); (2, 0); (3, 2); (3, 1); (2, 1); (3, 2); (2, 2);
      (2, 3); (2, 2); (2, 1); (2, 0); (1, 0); (3, 2); (3, 1); (2, 1); (3, 2);
      (2, 2); (2, 3); (2, 2); (2, 1); (1, 1); (3, 2); (2, 2); (2, 3); (2, 2);
      (1, 2); (2, 3); (1, 3); (1, 2); (1, 1); (1, 0); (0, 0); (3, 2); (3, 1);
      (2, 1); (3, 2); (2, 2); (2, 3); (2, 2); (2, 1); (1, 1); (3, 2); (2, 2);
      (2, 3); (2, 2); (1, 2); (2, 3); (1, 3); (1, 2); (1, 1); (0, 1); (3, 2);
      (2, 2); (2, 3); (2, 2); (1, 2); (2, 3); (1, 3); (1, 2); (0, 2); (2, 3);
      (1, 3); (0, 3); (0, 2); (0, 1); (0, 0)] *)

(* Imperative attempt also failed. I haven't looked further in to this as
   I don't intend on using 'for' and would prefer a recursive function *)
let add_incremented_tuples_to_list x x_max y y_max l =
  for x = x to x_max do
    for y = x to y_max do
      let l = ((x, y) :: l)
    done
  done
  l

let list_of_incremented_tuples = add_incremented_tuples_to_list 0 2 0 2 []